1. Field of the Invention
The present invention relates to an amplification optical fiber for amplifying signals of a plurality of channels, a fiber optic amplifier including the amplification optical fiber, an optical transmitter including the fiber optic amplifier, and an optical communication system including the optical transmitter.
2. Related Background Art
A fiber optic amplifier is an optical device for amplifying multiplexed signals (WDM signals) of a plurality of channels with different wavelengths at once through an amplification optical fiber to which pumping light is supplied in WDM (Wavelength Division Multiplexing) transmission, and is provided in an optical transmitter, relay device, or optical receiver in an optical communication system. As a general amplification optical fiber, a silica-based optical fiber (EDF: Erbium-Doped Fiber) having a core region doped with Er is known. A fiber optic amplifier (EDFA: Erbium-Doped Fiber optic Amplifier) using such an EDF as an amplification optical fiber amplifies signals in the C band (wavelength band: 1,530 to 1,560 nm) or L band (wavelength band: 1,570 to 1,600 nm) using pumping light having a wavelength of 980 nm or 1,480 nm.
An EDFA for amplifying C-band signals is described in, e.g., reference 1 xe2x80x9cTashiro et al., xe2x80x9cHigh-Power constant polarization mode dispersion-shifted Er-doped optical fiber amplifierxe2x80x9d, 1996, IEICE General Conference, C-330xe2x80x9d or reference 2 xe2x80x9cIzoe et al., xe2x80x9c980 nm Pumping Polarization Holding Er-Doped Optical Fiberxe2x80x9d, 2000, IEICE General Conference, C-3-26xe2x80x9d. Each of the EDFAs described in these references uses, as an amplification optical fiber, an EDF having a polarization maintaining structure for maintaining the polarized state of propagated light and amplifies C-band signals without changing the polarized state. The core region of the EDF described in reference 1 is doped not only with erbium (Er) at a concentration of 244 ppm but also with aluminum (Al) at a concentration of 600 ppm. The core region of the EDF described in reference 2 is doped not only with Er at a concentration of 1,210 wt.ppm but also with Al at a concentration of 11,600 wt.ppm.
Such an EDFA for amplifying signals while keeping the polarized state unchanged is arranged in, e.g., an optical transmitter which sets and outputs WDM signals of a plurality of channels in one of two polarized states that are orthogonal to each other. In this case, the EDFA is used to compensate for a transmission loss in a dispersion compensator in the optical transmitter, and the necessary gain is generally relatively as small as about 15 dB. Even when the EDFA has wavelength dependence of the gain (gain uniformity), the levels of WDM signals output from the optical transmitter are adjusted to a predetermined level by adjusting the output levels of light sources for outputting the signals of the channels. Hence, a gain equalizer is not always needed.
The present inventors examined the above-described prior arts and found the following problem. In the conventional optical transmitter, the output level of a general DFB-LD (Distributed FeedBack Laser Diode) used as a light source is adjusted by adjusting the supplied current value, though the output waveform changes simultaneously. In D-WDM (Dense Wavelength Division Multiplexing) transmission, the allowable variation width of a signal wavelength per channel is generally 0.1 nm, and a corresponding adjustable width of the output level of a DFB-LD is 2 dB.
Hence, in such D-WDM transmission, an EDFA installed in an optical transmitter preferably has a small gain uniformity (a sufficient gain spectrum flatness is ensured in the use wavelength band) and more preferably has a gain uniformity of 2 dB or less. In this case, a gain equalizer must be arranged in the EDFA. However, a gain equalizer having a polarization maintaining function is expensive and also degrades the polarization extinction ratio.
The present invention has been made to solve the above-described problem, and has as its object to provide an amplification optical fiber (EDF) which requires no gain equalizer and has a small gain uniformity (flat gain spectrum) between signal channels and high polarization extinction ratio, a fiber optic amplifier (EDFA) including the amplification optical fiber, an optical transmitter including the fiber optic amplifier, and an optical communication system including the optical transmitter.
An amplification optical fiber according to the present invention is a silica-based optical fiber for amplifying WDM signals of a plurality of channels with different wavelengths at once upon receiving pumping light. The amplification optical fiber has a polarization maintaining structure for maintaining the polarized state of signals. In addition, erbium (Er) and aluminum (Al) at a concentration of 4 wt % or more are doped into at least part of the light propagation region through which the signals propagate. A fiber optic amplifier according to he present invention comprises the amplification optical fiber (amplification optical fiber according to the present invention) for amplifying signals upon receiving pumping light, and a pumping light source for supplying the pumping light to the amplification optical fiber. When pumping light (the wavelength is, e.g., 1,480 or 980 nm) is supplied to the amplification optical fiber, signals (e.g., C band or L band) input to the amplification optical fiber are amplified while keeping their polarized state.
The amplification optical fiber applied to the fiber optic amplifier according to the present invention has a polarization maintaining structure and also a structure in which Er and Al at a concentration of 4 wt % or more are doped into at least part of the core region included in the light propagation region. For this reason, a flat gain spectrum and high polarization extinction ratio can be obtained without using a gain equalizer. In addition, since this amplification optical fiber requires no gain equalizer, the manufacturing cost can be reduced.
The amplification optical fiber according to the present invention preferably has a cutoff wavelength of 1.15 xcexcm or more. The fiber optic amplifier according to the present invention preferably amplifies signals with wavelengths included in the L band by the amplification optical fiber. In this case, even when the amplification optical fiber is wound in a coil shape and accommodated, the bending loss of the amplification optical fiber falls within the allowable range. Hence, even when the amplification optical fiber becomes long to amplify L-band signals, the fiber optic amplifier can be made compact, and any increase in bending loss can be effectively suppressed.
In the fiber optic amplifier according to the present invention, the polarization extinction ratio at a wavelength of 1.60 xcexcm (1,600 nm) is preferably 20 dB or more, and more preferably, 25 dB or more.
An optical transmitter according to the present invention comprises a first light source system for outputting signals of a plurality of channels with different wavelengths, each signal being set in a first polarized state, and a second light source system for outputting signals of a plurality of channels with wavelengths different from those of the signals output from the first light source system, each signal being set in a second polarized state different from the first polarized state. Especially, the signal channels output from the optical transmitter are sequentially alternately assigned to the first and second light source systems such that the polarized state alternately changes from the short wavelength side to the long wavelength side so as to effectively suppress any nonlinear optical phenomenon such as four wave mixing in the transmission path. The optical transmitter has a multiplexer for multiplexing the signals output from the first and second light source systems while keeping their polarized states unchanged. The optical transmitter also has a fiber optic amplifier having the above-described structure. The fiber optic amplifier may be arranged, e.g., at a position to amplify the signals output from the first and second light source systems at once.
The optical transmitter according to the present invention may have a structure for compensating for chromatic dispersion every channel or every several channels (a dispersion compensation fiber is inserted between the first light source system and the multiplexer). Actually, since the necessary dispersion compensation amount changes for each channel, the length of the dispersion compensation fiber to be installed also changes for each signal channel. However, since the dispersion compensation fiber has a large transmission loss, the signal level difference between a signal channel for which a dispersion compensation fiber is prepared and a signal channel for which no dispersion compensation fiber is prepared increases. To prevent this problem, for the optical transmitter, one or a plurality of fiber optic amplifiers each having the above-described structure may be inserted between the first light source system and the multiplexer for a corresponding signal channel. One or a plurality of fiber optic amplifiers each having the above-described structure may also be inserted between the second light source system and the multiplexer to uniform the signal levels between the signal channels.
An optical communication system according to the present invention comprises an optical transmitter having the above-described structure and transmits WDM signals output from the optical transmitter.
In such an optical communication system, each WDM signal output from the first or second light source system while being set in the first or second polarized state, and is multiplexed. Between the first or second light source system and the multiplexer, to improve the variation in signal level between channels due to dispersion compensation executed for each signal channel, the signals of some channels are amplified by a fiber optic amplifier and output from the optical transmitter. With this arrangement, the signal levels between the signal channels output from the optical transmitter can be uniformed. In addition, since the WDM signals output from the first and second light source systems are set in the two polarized states that are orthogonal to each other, any nonlinear optical phenomenon during propagation through the fiber transmission path can be effectively suppressed, and the wave spacing between the WDM signals can be reduced (the signal transmission amount can be increased). In one of the first and second light source systems, the signals of some channels are amplified by a fiber optic amplifier having a flat gain spectrum and high polarization extinction ratio, and the variation in gain between the signal channels can be effectively reduced. Even in respect to this point, the wave spacing between the signal channels can be reduced (the signal transmission amount can be further increased).
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.